Oriented polymer films, a process for the preparation thereof

ABSTRACT

A process for the continuous preparation of an oriented film of a polymer material comprising shearing at least one film of a solution and/or gel of the polymer material between at least one set of two solid surfaces which are in motion relative to one another. At least one of which surfaces is endless, so as to cause by the shearing, solidification of the polymer material from the solution or gel to a degree sufficient that a coherent, oriented film of the polymer material is formed on one of the surfaces. The film is sufficiently coherent so as to be capable of being removed from the surface on which it is formed in a continuous manner.

This invention relates to oriented polymer films; more particularly,this invention relates to processes for the continuous preparation oforiented films of polymer materials; to apparatus which may be used insuch processes; to oriented films of polymer materials so prepared andto certain oriented films as new products; and to composite materialscomprising the oriented films.

It is known that a dilute solution of linear polyethylene of very highmolecular weight (for example, M_(w) about 1.5×10⁶) in xylene at atemperature up to about 125° C. will form a fibre when seeded from asurface in contact with the solution when this solution is subjected toCouette flow. (A. Zwijnenburg and A. J. Pennings, Colloid & Polymer Sci:254, p. 868 (1976).) The fibre can be continuously removed as a threadline and reeled. Such fibres are found to have very high ultimatetensile strengths and Young's moduli (typically 1 GPa and 100 GPa,respectively).

More recently, it has been shown that by allowing such a fibre to seedfrom a rotating conical surface it may grow in a helical coil around thesurface and will self-coalesce to form a ring of linear polyethylenefilm from the polyethylene solution. (A. Coombes and A. Keller, J.Polym. Sci. Polym. Phys. Ed., 17 p. 1637 (1979).) Furthermore, it hasalso been shown that the incorporation of polypropylene into thesolution can similarly lead to films of a polymer material which is avarying blend of linear polyethylene and polypropylene. (A. Coombes, C.G. Cannon and A. Keller, J. Polym. Sci. Polym. Phys. Ed., 17 p. 1957(1979).)

This invention seeks to provide a process for the continuous preparationfor oriented films of polymer materials which is more suited tocommercial production.

According, therefore, to one aspect of this invention, there is provideda process for the continuous preparation of an oriented film of apolymer material, which process comprises:

(i) shearing at least one film of a solution and/or gel of the polymermaterial between at least one set of two surfaces, at least one of whichsurfaces is endless and in relative motion; and

(ii) continuously removing the or each oriented film of polymer materialfrom one of the surfaces on which it is formed.

By "film of solution and/or gel" is meant herein a thin layer, typicallyof a thickness not greater than 3 mm.

The process of the present invention is applicable to any thermoplasticorganic polymer material, preferably a crystallisable such polymermaterial, provided only that the polymer material comprises at least afraction of sufficiently high molecular weight and that this lattermaterial can form a solution and/or gel. Examples include linear vinylhydrocarbon polymers, polyethylene oxide, polyacetals such aspolyoxymethylene and polyacetaldehyde, aliphatic polyamides, polyesterssuch as polyethylene terephthalate and fluorinated polymers such aspolyvinylidene difluoride.

Preferably, the polymer material comprises a polyolefin, for examplepolyethylene, polypropylene, a polyethylene-polypropylene copolymer or apolyolefin blend comprising at least one thereof, especially linearpolyethylene.

It is desirable that the polymer material has a weight average molecularweight (M_(w)) greater than about 500,000, desirably greater than about600,000. Below this value there is not sufficient of the polymermaterial of sufficiently high molecular entanglement and/or relaxationtime to enable highly oriented films, in which the polymer material isbelieved to exist as extended chain fibrils, to be formed by the processof the present invention with commercial viability, if at all.Preferably, M_(w) is greater than about 750,000, desirably greater thanabout 850,000, especially greater than about 1,000,000.

The film of solution and/or gel of the polymer material is preferablyless than 3 mm thick, especially less than 2 mm thick. It isparticularly preferred that the film is 1 mm thick or less; for example,no greater than 0.5 mm.

It is highly desirable, and usually necessary, to maintain the solutionand/or gel of polymer material at an elevated temperature; for example,it is desirable to maintain the solution and/or gel at a temperature notless than 50° C., and preferably not less than 30° C., below the meltingtemperature of the polymer material. In the case of linear polyethylene,it is desirable that the solution and/or gel is maintained, at standardpressure, at a temperature greater than 90° C., desirably greater than100° C., preferably at a temperature greater than 110° C., desirablygreater than 120° C., such as up to 130° C. The solution and/or geltemperature should not exceed the melting temperature of the polymermaterial. The tensile properties of the oriented film are found toincrease as the solution and/or gel temperature increases. However, themass of oriented film produced per unit time is found to decrease withincreasing solution and/or gel temperature but to increase withincreasing shear. To obtain maximum mass per unit time of the orientedfilm it is, therefore, necessary to use as low a solution and/or geltemperature as is consistent with the required mechanical properties inconjunction with high shear. The mass growth rates exhibited are muchgreater than those found in equivalent fibre preparations. It isbelieved that the presence of a second surface enhances the mass growthrate (at a given temperature) by defining and increasing the effectivewidth over which growth can occur and possibly also by providingenhanced shear between the surfaces.

The solvent used will depend on the nature of the polymer material. Inthe case of polyolefins, such as linear polyethylene, a hydrocarbon, forexample an aromatic hydrocarbon boiling, at standard pressure, above140° C. such as a xylene, may be used. For polyethylene oxide polarsolvents, for example, water or chlorinated hydrocarbons, may be used.Solutions and/or gels of the polymer material having a concentration forexample from 0.1 wt% to 10 wt%, preferably from 0.2 wt% to 5 wt% aredesirable.

Desirably, the surface on which the oriented film of polymer material isformed is at least in part wetted and adhered to by the solution and/orgel of the polymer material. However, the oriented film so formedshould, in general, be sufficiently mobile to facilitate ready strippingtherefrom during its removal. The surface may be of the same ordifferent polymer material, for example PTFE, or of a metal. The textureof the surface is also an important factor in determining whether theabovementioned functional requirements are met. Thus, where a surface offabric, such as "leno" woven cotton, is used it is usually found thatthe oriented film is irremovably impregnated therein. (Such a materialmay, nonetheless, be of value as a composite material.) At the otherextreme, where the surface is substantially completely not adhered toand smooth, such as a PTFE sheet, no film is usually formed. Theseeffects can be utilised to good effect in accordance with a preferredcompromise feature of this invention wherein a portion of, preferablythe majority of the, or one of the, surfaces on which the oriented filmof the polymer material is formed is smooth and not adhered to while aportion, preferably the minority, is adhered to by the solution and/orgel. Specifically, a smooth sheet, for example of PTFE, which is notadhered to can comprise a plurality of lines, for example two lines ofstitches, for example of cotton thread, which extend in the machinedirection. It is preferred that at least the or each surface on whichthe oriented film of the polymer material is formed is in motion.

It is also found, in accordance with a particularly preferred feature ofthis invention, that a relatively coarsely woven fabric such as a wovenfabric may be used as a surface from which to form a harvestableoriented film of polymer material. The material known as "VELCRO"(manufactured in accordance with U.S. Pat. Nos. 2,717,434 and 3,009,235and GB Pat. Nos. 1289825, 1295069, 1299897, 1345607 and 2009257;"VELCRO" is a registered Trade Mark) has been formed to be particularlysuitable in this respect. Surprisingly, the continuous oriented films soproduced show little or no tendency to lateral shrinkage on drying.

This invention also provides a process according to any preceding claimwherein a plurality of lines of oriented films of a polymer material iscontinuously prepared.

The or each oriented film of polymer material may be continuouslyremoved in a manner known per se; for example using the haul-off shownin FIG. 5 of the accompanying drawings. It is found in practice thathaul-off load is a good indicator of film thickness (that is, thatgrowth occurs at constant stress).

According to a further aspect of this invention, there is providedapparatus for the continuous preparation of an oriented film of apolymer material, which apparatus comprises at least one set of twoadjacent surfaces, at least one of which surfaces is endless, movablerelative to each other; and means for continuously removing the or eachoriented film of polymer material from one of the surfaces on which itis formed. Preferably, such apparatus additionally comprises one or morebaths in which the or each set of adjacent surfaces is contained.

Desirably, the, or one of the, endless surfaces in the, or one of the,sets comprises a belt of a cylinder. Increase in diameter of thecylinder, as in scale-up, will increase the defined contact area andpossibly also the shear and thus, as mentioned earlier, the mass oforiented film produced per unit time. The other surface may comprise achisel-edged seed-rod of the polymer material or blade or metal.However, it is preferred that the two surfaces in the, or one of the,sets are endless.

The machine direction of the oriented film of the polymer material, thedirection of one surface, for example the belt and the shear axis of theother surface for example, the cylinder in a set are independentlyvariable. When none of these is normal to any other the oriented film isoriented in a direction different from the machine direction and thedegree of orientation changes continuously across the film. Where inthe, or one of the, sets the machine direction of the oriented film ofpolymer material is normal to one surface but not normal to the shearaxis of the other surface, the oriented film is oriented in the machinedirection but the degree of orientation changes continuously across thefilm. Where in the, or one of the, sets the machine direction of theoriented film of polymer material is normal both to one surface and tothe shear axis of the other surface, the oriented film is oriented inthe machine direction and the degree of orientation does notsubstantially change across the film. Preferred apparatus as hereindescribed is that wherein in the, or one of the, sets one surfacecomprises a belt and the other surface comprises a right cylinderrotatable about its axis and bounded, at least in part, by the belt.

With such apparatus there are, in principle, five modes of operation:

(a) a moving cylinder and stationary belt;

(b) a moving cylinder and a belt moving in the opposite hand;

(c) a moving cylinder and a belt moving in the same hand but slower;

(d) a moving cylinder and a belt moving in the same hand but faster;

(e) a stationary cylinder and a moving belt.

In practice, (a) does not appear to produce continuous lengths ororiented film; indeed, it has been found that, using a VELCRO surface, abelt tension of at least 20N, and preferably greater than 25N isrequired to produce an oriented film.

This invention also provides an oriented film of a polymer material,especially linear polyethylene or polypropylene or a blend thereof,prepared by a process, or in apparatus, as herein described. Moreparticularly, this invention provides a continuous length of an orientedfilm of linear polyethylene, especially with M_(w) greater than 500,000,which has a Young's modulus greater than 40 GPa, preferably greater than50 GPa. The Young's modulus is suitably measured along the machinedirection; the direction in which the oriented film is formed.

This invention further provides a continuous length of an oriented filmof polypropylene, especially with M_(w) greater than 500,000 which has aYoung's modulus greater than 20 GPa. It may be advantageous for theoriented film to be fibrillated.

In accordance with a further aspect of this invention, there is provideda composite material the dispersed phase of which comprises an orientedfilm of a polymer material as herein described. The continuous phase maycomprise a cement such as a Portland or pozzolanic cement, athermosetting polymer matrix such as an epoxy resin, or a thermoplasticmatrix such as a polyolefin, for example polyethylene, polypropylene ora blend thereof. It may be advantageous for the oriented film to becomminuted prior to incorporation in the continuous phase.

The invention will be further described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 represents apparatus according to the invention in which onesurface is provided by a chisel-shaped seed-rod and the other by arotatable cylinder;

FIG. 2 represents apparatus according to the invention in which onesurface is provided by a scraper blade and the other by a rotatablecylinder;

FIG. 3 represents apparatus according to the invention in which onesurface is provided by a belt and the other by a rotatable cylinder;

FIG. 4 represents apparatus according to the invention in which onesurface is provided by a chisel-shaped seed-rod and the other by arotatable cylinder of large (27 cm diameter compared with 8 cm diameter)diameter;

FIG. 5 represents apparatus according to the invention in which onesurface is provided by a belt and the other by a rotatable cylinder oflarge diameter; and

FIGS. 6 to 10 inclusive schematically represent other surfaceconfigurations.

In the drawings, the apparatus represented in FIGS. 1 and 4 comprises abath 1 in which a cylinder 2 having a cylindrical surface of PTFErotatable about a horizontal axis is contained. A chisel-shaped seed-rod3 of polyethylene is maintained in sliding contact with the cylinder.

In FIG. 2 the seed-rod is replaced by a scraper blade 4 of sprung steel.

In FIGS. 3 and 5 the seed-rod is replaced by a continuous belt 5 of PTFEwhich comprises two lines 6 of stitched cotton thread which is insliding contact with the cylinder and which is friction drivable bypassage over drivable roller(s) 7. In FIG. 3 there is also representedbelt-tensioning spools 8 over which the belt is laced and a filmwind-off spool 9.

In use, the bath 1 is substantially filled with a dilute (less than 10wt%) solution and/or gel of high molecular polyethylene in xylene whichis equilibriated at about 110° C. by heating means (not shown). Thecylinder is then caused to rotate by drive means (not shown). Thereafterthe seed-rod, scraper blade or belt is maintained in sliding contact(the belt optionally being driven) with the cylinder. As film 10 isformed it is laced to a wind-off spool.

The following Examples illustrate the invention.

EXAMPLE 1

5 g of high molecular weight linear polyethylene ("Hostalen GUR 415 exHoechst) was added, together with 5 g of antioxidant ("Topanol" C.A. exI.C.I.), at room temperature to 1,000 cm³ of xylene (technical grade) ina "Thermomix" heater/blender. The heater/blender had been adapted byinclusion of a nitrogen purge and a "Eurotherm" temperature controller;it was also solvent proofed. The heater/blender was then switched on andallowed to heat up to 128° C. while being agitated for about 10 minutes.Mixing was continued for a further 30 minutes at this temperature beforethe resulting solution and/or gel was transferred to the apparatus forprocessing. All of the solution and/or gel preparation was carried outunder a nitrogen blanket.

The processing apparatus was essentially as shown in FIG. 1 of theaccompanying drawings. The rotatable cylinder had a diameter of 8 cm andthe solution and/or gel was maintained therein at a temperature of 110°C. The rotatable cylinder was then set in motion with a surface linearvelocity of 8.5 m.min⁻¹. After 5 minutes a chisel-shaped seed-rod ofpolyethylene was inserted into the bath and maintained in slidingcontact with the cylinder. Film began to grow on the cylinder surfacewith a width essentially the same as that of the seed-rod. The filmappeared to form by the simultaneous, parallel growth across the wholewidth of the seed-rod (and not as a single fibril coiling repeatedlyabout the cylinder).

The experiment was repeated using conditions shown in the Table.

                                      TABLE                                       __________________________________________________________________________                                Belt                                                                             Belt                                                                              Belt                                                       Seed-rod                                                                           Scraper-blade                                                                        (a)'                                                                             (b) (c)                                        __________________________________________________________________________    Cylinder speed                                                                        8 cm diameter                                                                         8.5  13.5   8.5                                                                              8.5 8.5                                        (m. min.sup.-1)                                                               Belt speed                                                                            8 cm diameter                                                                         --   --     0  -0.9                                                                              +0.9                                       (m. min.sup.-1)                                                               Cylinder speed                                                                        27 cm diameter                                                                        27.2 --     18 --  18                                         (m. min.sup.-1)                                                               Belt speed                                                                            27 cm diameter                                                                        --   --     0  --  +0.9                                       (m. min.sup.-1)                                                               __________________________________________________________________________

In the case of belt-induced growth a belt of PTFE of about 46 cmcircumference and 2.6 cm width and sewn with cotton as aforesaid wasused with the 8 cm diameter cylinder while with the 27 cm diametercylinder the belt was about 100 cm circumference and 1.8 cm width.

Temperature could range from 90° to 130° C.; cylinder speed from 0 to100 m.min⁻¹ and belt speed (where applicable) from 0 to ±100 m.min⁻¹.

The films produced had a nominal tensile strength of 1.24 GPa and aninitial Young's modulus of 41 GPa. (It is believed that, as the film isproduced in a more homogeneous manner, these values will increase.)

EXAMPLE 2

Proceeding in essentially the manner of the preceding Example bututilising the apparatus shown in FIG. 5 of the accompanying drawingswith a "VELCRO"-coated belt at a temperature of 110° C. and a cylinderspeed of 10.8 m.min⁻¹ and a belt speed of 0.3 m.min⁻¹ of opposite hand(mode b) a 2.1 m length of tape was harvested from a belt 1.7 m inlength.

We claim:
 1. A process for the continuous preparation of an orientedfilm of a polymer material, which process comprises:(i) shearing atleast one film of a solution and/or gel of the polymer material betweenat least one set of two solid surfaces which are in motion relative toone another and which in a zone in which said shearing takes place,co-extend in closely adjacent face-to-face relationship, at least one ofwhich surfaces is endless, so as to cause by said shearing,solidification of said polymer material from said solution or gel to adegree sufficient that a coherent, oriented film of said polymermaterial is formed on one of said surfaces, which film is sufficientlycoherent to be capable of being removed from the surface on which it isformed; and (ii) continuously removing the or each oriented film ofpolymer material from the surface on which it is formed.
 2. The processof claim 1 wherein said polymer material is a polyolefin.
 3. The processof claim 2 wherein said polyolefin is selected from the group consistingessentially of polyethylene, polypropylene, a polyethylene-polypropylenecopolymer or a polyolefin blend comprising at least one thereof.
 4. Theprocess of claim 3 wherein said polyethylene is linear polyethylene. 5.The process of claim 4 wherein said polymer material has a weightaverage molecular weight greater than 500,000.
 6. The process of claim 5wherein said polymer material has a weight average molecular weightgreater than 750,000.
 7. The process of claim 6 wherein saidpolyethylene is a linear polyethylene having a weight average molecularweight greater than 1,000,000.
 8. The process of claim 7 wherein the oreach oriented film of solution and/or gel of the polymer material has athickness no greater than 2 mm.
 9. The process of claim 8 wherein thesolution of the polymer material is maintained, at standard pressure, ata temperature greater than 90° C.
 10. The process according to claim 9wherein the solution is maintained at standard pressure, at atemperature greater than 110° C.
 11. The process of any preceding claimwherein said solution further comprises a hydrocarbon having a boilingpoint above 140° C. at standard pressure.
 12. A process according to anypreceding claim wherein the concentration of the polymer material in thesolution is from 0.1 wt% to 10 wt%.
 13. A process according to anypreceding claim wherein the, or one of the, surfaces on which theoriented film of polymer material is formed is at least in part adheredto by the solution and/or gel of the polymer material.
 14. A processaccording to claim 13 wherein a portion of the, or one of the, surfaceson which the oriented film of the polymer material is formed is smoothand not adhered to while a portion is adhered to by the solution and/orgel.
 15. A process according to any preceding claim wherein at least theor each surface on which the oriented film of the polymer material isformed is in motion.
 16. A process according to any preceding claimwherein a plurality of lines or oriented films of a polymer material iscontinuously prepared.